Babesia divergens is a tick transmitted hemoprotozoan parasite that causes human Babesiosis in Europe. Neither B. divergens nor its vector tick are indigenous to the U.S. However, a recent case of acute Babesiosis in a resident of Kentucky was shown to be caused by a parasite morphologically indistinguishable from European Babesia divergens. The parasite in the Kentucky case was found to differ in the small subunit ribosomal RNA gene sequence at only 3 base positions from that of European B. divergens, providing evidence of conspecifity. Recently a Babesia isolate from cottontail rabbits on Nantucket Island was reported to have an identical gene sequence to that of the Kentucky parasite. Molecular epidemiology has also identified this parasite in Ixodes ticks on Nantucket Island, suggesting that an enzootic situation exists. [unreadable] [unreadable] The identification of another Babesia sp. in the US capable of causing human babesiosis is of major concern. Two other Babesia species that cause human Babesiosis in the US, Babesia microti, occurring mostly in the northeastern U.S., and the WA1 isolate, occurring in Washington state and California, have been implicated in blood transfusion acquired cases of babesiosis. Thus, the finding that another previously unknown human blood pathogen is troublesome. [unreadable] [unreadable] The purpose of this study is to compare the culture requirements and target genes of the Kentucky and Nantucket Babesia isolates with European B. divergens (Purnell strain) to determine if conspecificity exists among these parasites. Specific aims include establishing in vitro cultures of the Nantucket isolate and molecular comparison of ITS, HSP70, and cysteine protease genes from European B. divergens with those of the Kentucky and Nantucket isolates. Cultures of the Nantucket isolate will provide an unlimited source of parasites to complete these goals and for future studies. The molecular aspects may provide information that will be useful in future efforts to devise diagnostic tests. [unreadable] [unreadable]